Projection television systems employing LCDs or DMDs are currently under development for use as, among other things, computer monitors. Such projection televisions preferably employ a single lens system which forms an image of either a single panel having, for example, red, green, and blue pixels, or three individual panels, one for each color.
In either case, and, in particular, in the three panel case, the lens system normally needs to have a long aperture stop to object distance (ASOD) to accommodate the optical elements, e.g., filters, beam splitters, prisms, and the like, used in combining the light from the different color optical paths which the lens system projects towards the viewing screen.
The illumination of a pixelized panel plays an important role in the performance of projection TVs employing such panels. In particular, it is important to match the location and size of the exit pupil of the illumination system with the entrance pupil of the lens system to obtain a bright, uniformly-illuminated image on the TV screen. Since illumination optics generally work best when the exit pupil is located a long distance from the light source, it is desirable to use a projection lens system with a long entrance pupil distance. Also, LCD panels work best when light passes through them at small angles.
Telecentric lens systems are systems which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinitum. Since light can propagate through a lens system in either direction, the pupil at infinity can serve as either an entrance or an exit pupil depending upon the system's orientation with respect to the object and the image. Accordingly, the term "telecentric pupil" will be used herein to describe the system's pupil at infinity, whether that pupil is functioning as an entrance or an exit pupil.
In practical applications, the telecentric pupil need not actually be at infinity since a lens system having an entrance or exit pupil at a sufficiently large distance from the system's optical surfaces will in essence operate as a telecentric system. The principal rays for such a system will be substantially parallel to the optical axis and thus the system will in general be functionally equivalent to a system for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the term "telecentric lens system" is intended to include lens systems which have at least one pupil at a long distance from the lens elements, and the term "telecentric pupil" is used to describe such a pupil at a long distance from the lens elements. For the lens systems of the invention, the telecentric pupil distance will in general be at least about 10 times the system's focal length.
In addition to having a long ASOD and a telecentric pupil, lens systems for use with pixelized panels generally need to have a high level of aberration correction, including lateral color correction. Lateral color, i.e., the variation of magnification with color, manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection televisions employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With pixelized panels, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible.
Accordingly, a higher level of lateral color correction is needed from the lens system. In particular, for a VGA computer monitor, the lateral color evaluated across the entire active surface of the pixelized panel(s) over the visual light spectrum should be less than about the diagonal of a pixel and preferably less than about 1/2 the diagonal of a pixel.
Pixelized panels and their use in computer monitor applications also lead to stringent requirements regarding the correction of distortion and the attainment of flat field imagery, i.e., the achieving of a high level of correction of the field curvature of the lens system. This is so because when viewing data displays, good image quality is required even at the extreme points of the field of view of the lens system. Similarly, it is also important to keep an even illumination level across the image of the pixelized panel, i.e., to maintain the smallest relative illumination fall-off possible due to vignetting in the lens system. Further, various illumination schemes may require lens systems having large numerical apertures, e.g., apertures corresponding to a f-number of 2 or faster.
For rear projection applications, it is desirable to have as small an overall package size (monitor size) as possible. In terms of the optics, this means that the imaging conjugates should be made as small as possible while still maintaining a large image size. This, in turn, means that the projection lens system should have a wide field of view, e.g., preferably a field of view whose half angle is at least about 25.degree. or higher. A lens system having such a field of view is referred to herein as a "wide angle" system.
The lens systems described below address all the above requirements and can be successfully used in producing projection televisions and, in particular, computer monitors, where a high quality color image is required.